This invention relates to rubbery polymers having an unusually high concentration of reactive carboxylic acid groups on the surface of the polymer particles. More particularly, it relates to core-shell polymer particles having soft polymers, such as polybutadiene or styrene-butadiene copolymer, as the core particles and a shell comprising poly(methylmethacrylate) and a copolymer of methylmethacrylate and methacrylic acid.
Polymers containing "available" carboxylic acid groups for reaction are valuable industrial materials. In particular, soft polymers, particularly rubbery polymers having such carboxyl groups can be utilized in various applications such as, for example, in adhesives, binders, coatings, etc. However, water-based dispersions of these rubbery polymers, such as, styrene-butadiene copolymers, having carboxylic acid concentration higher than about 3 or 4 percent are extremely unstable.
Efforts at obtaining water-based dispersions of styrene-butadiene copolymers with higher carboxylic acid concentration have heretofore met with little success. In pending U.S. Patent application Ser. No. 115,824, filed Jan. 28, 1980, there are disclosed core-shell polymer compositions in which the core comprises soft rubbery polymer particles such as polybutadiene or styrene-butadiene copolymer, and the shell comprises a polymerized alkyl methacrylate, notably poly(methylmethacrylate). Latices of these core-shell copolymers were found to exhibit excellent stability. However, efforts to incorporate significant amounts of carboxylic acid groups on the copolymer shell were not successful. For example, an attempt to graft 10 percent (by weight) of an equimolar mixture of methylmethacrylate and methacrylic acid onto SBR core particles was not successful. The latex coagulated into a rubbery mass which could not be redispersed by mechanical agitation. It was apparent that the SBR core was involved in the coagulation and that, in all likelihood, the monomers diffused into the core before they could have been polymerized.
It is therefore a prime object of this invention to develop a stable aqueous dispersion of a soft polymer, particularly SBR having a high amount of carboxylic acid groups. It is a further object of this invention to develop a relatively simple process for the preparation of such dispersions.